Smooth Bottom Net Trawl Fishing Gear Effect on - New England ...

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NOAA/NMFS Unallied Science Project, Cooperative Agreement NA16FL2264 May 2005 <strong>Smooth</strong> <strong>Bottom</strong> <strong>Net</strong> <strong>Trawl</strong> <strong>Fishing</strong> <strong>Gear</strong> <strong>Effect</strong> on the Seabed: Investigation of Temporal and Cumulative <strong>Effect</strong>s BKAM/CR 3.2 Geophysical Results The study sites Mud Hole and Little Tow as described in our 2003 NOAA report, “Near Term Observations of the <strong>Effect</strong>s of <strong>Smooth</strong> <strong>Bottom</strong> <strong>Net</strong> <strong>Trawl</strong> <strong>Fishing</strong> <strong>Gear</strong>,” are approximately 10 km offshore of Scituate, MA, south of Boston. Mud Hole, the slightly deeper and larger outer basin, has more soft sediment. The slightly shallower and narrower Little Tow basin appears a higher energy environment with coarser, sorted material. During major winter storm events, energy from large swells penetrate deeply enough to disturb the substrate of both basins. The sections that follow describe in detail the results from a more detailed bathymetric survey conducted in September 2002 and substrate characteristics detected by side-scan sonar and grain size analysis before (July 2002) and after chronic trawling (October and November 2002) in the study sites Mud Hole and Little Tow. 3.2.1 Bathymetric Results Figures 3.2.1-1 and 3.2.1–2 are bathymetric surface maps of Mud Hole and Little Tow, respectively. Depths at Mud Hole ranged from 119.5 to 142.7 ft below Mean Lower Low Water (MLLW). The mean depth was 136.2 ft below MLLW. Mud Hole slopes from north to south and the majority of the site contains very little relief. The northern portions of Lanes 1 and 2 are located on a gradual slope (approximately 7 ft change in elevation over 1,000 ft) that originates at a rock outcrop to the north. Sample station MH-1A is located on the lower portion of this slope, and Stations MH-1B, MH-2A and MH-2B are near the toe of this slope. The remaining sample stations were on relatively flat portions of the seafloor. Depths at Little Tow ranged from 113.7 to 126.8 ft below MLLW. The mean depth was 121.2 ft, fifteen feet shallower than the mean depth of Mud Hole. The terrain of Little Tow was more irregular than that of Mud Hole. A shallow depression is located along the northern portions of Little Tow Lanes 1 and 2. Sample station LT-2A was located within this depression. Another larger depression was observed at the northern end and immediately east of Lane LT-4. Sample station LT-4A was located within this depression. 3.2.2 Side-Scan Sonar Results The late July pre-chronic trawling, and September and November 2002 post-chronic trawling side-scan sonar surveys were conducted in order to: (1) Map the presence of project-related gear disturbances on trawled lanes; (2) Confirm the absence of project-related gear disturbances on control lanes; (3) Document non-project related background disturbances associated with commercial fishing; and (4) Document physical changes to the seabed over the course of the four-month study. 19
NOAA/NMFS Unallied Science Project, Cooperative Agreement NA16FL2264 May 2005 <strong>Smooth</strong> <strong>Bottom</strong> <strong>Net</strong> <strong>Trawl</strong> <strong>Fishing</strong> <strong>Gear</strong> <strong>Effect</strong> on the Seabed: Investigation of Temporal and Cumulative <strong>Effect</strong>s BKAM/CR Both non-projected ‘raw’ sonar data and geographically referenced data were inspected to meet these goals. Geo-referenced side-scan sonar imagery was imported to ArcView GIS for detailed inspection. Data layers representing video drifts (Section 3.3), benthic grab samples (Section 3.4) and SPI observation points (Section 3.5) were added to the GIS project to aid interpretation. The high-resolution non-projected “waterfall” side-scan imagery often provides clearer bottom imagery, and was closely inspected. Side-scan sonar data is typically depicted as a range of grey shades that correspond to the strength of the returning acoustic signal. The eye can perceive a wider range of color shades than grey shades (Fish & Carr, 2001). Recognition of targets and fine bottom features in sonar data may be facilitated by inverted colorized data displays. A key to the shading and color ranges employed for this trawl surveys are provided below. Index to Sonar Image Color Scales Sonar shadow------------ Weak Signal Return----------------------------Strong Signal Return In general, weak signal returns correspond to: • smooth seabed substrates (e.g., fine sediments with little microtopography), • to soft materials that absorb the signal, or • to a seabed sloping away from the signal source (towfish). Strong signal returns correspond to: • rough seabed substrates (e.g., gravel, cobble), • highly reflective materials, or • to a seabed sloping towards the signal source. Features that rise above the seabed (e.g., boulders) reflect more of the sonar energy than the surrounding substrate resulting in strong signal returns due to decreased angle of incidence. These features often prevent insonification of the area opposite the signal source, resulting in a sonar “shadow”. See Figure 3.2.2-1 on the following page for an example of shadowing behind the peaks of sand waves. The length of these shadows can often be used to calculate the approximate height of the elevated features. 20
Page 1: SMOOTH BOTTOM NET TRAWL FISHING GEA
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Maciolek, N.J., R.J. Diaz, D. Dahle
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Table 3.3-2. Organisms observed per
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Table 3.3-4. Organisms observed per
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Table 3.3-5. Comparison of 2001 ROV
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Station LT02-1B - P2 Trawl<
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Station LT02-3A Trawl</stro
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Station LT02-4A - P1 Control Lane O
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Trawl Station Resu
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Trawl Station Resu
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Trawl Station Resu
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Table 3.6-2 2002 Trawl</str
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Table 3.6-3 (continued) CRUSTACEA A
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Table 3.6-5 Summary of Blackback (W
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Table 3.6-5 (continued) OTHERS Sarc
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Table 3.6-7 Dominant Species in Sed
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FIGURES
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Figure 1.0-2. Smooth</stron
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Figure 1.0-4 Side-scan sonar base m
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Figure 3.2.1-2 Bathymetric surface
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Number/minute 4 3 2 1 0 July Octobe
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Number/minute Number/minute 4 3 4 2
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Number/minute Number/minute 30 20 1
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Number/minute Number/minute 2.5 2 1
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Number/minute Number/minute July -
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Figure 3.4-2 For key species, the a
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Figure 3.4-4 For key species, the a
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Figure 3.4-6: Cluster analysis of M
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Figure 3.4-9 Little Tow 2002 Cluste
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Figure 3.4-12a. Little Tow 2001 and
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Note: [LT02 = Little Tow 2002 sampl
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Results of 2002 REMOTS Surveys to E
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Kg/Tow 200 180 160 140 120 100 80 6
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Kg/1000m2 Kg/1000m2 7 6 5 4 3 2 1 0
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Kg per 1000m2 350 300 250 200 150 1
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Numbe of individuals Number of Indi
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Number of individuals Number of ind
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PHOTOGRAPHS
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Photograph 2.2-2 Christopher Dunbar
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a. Chris Dunbar sieving benthic sam
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Photograph 2.7-1 CR Environmental e
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a. Fisherman Andrew Mirarchi viewin
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2001 2002 Cobble Bottom</st
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MH-3A July 2002 Pre-Trawl</
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MH-4A July 2002- Pre-Trawl<
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LT-1B July 2002- Pre-Trawl<
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LT-3A July 2002- Pre-Trawl<
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LT-4A July 2002- Pre-Trawl<
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Cobble bottom at Little Tow Small r
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2001 2002 Flounder Rock Crabs Sea S
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